Parameter values for parameter (n_estimators) need to be a sequence

Question

I am getting the error, not sure how to fix it. Can you please help. The entire code can be found at https://github.com/kthouz/NYC_Green_Taxi/blob/master/NYC%20Green%20Taxi.ipynb

optimize n_estimator through grid search

def optimize_num_trees(alg,param_test,scoring_method,train,predictors,target):
    """
    This functions is used to tune paremeters of a predictive algorithm
    alg: sklearn model,
    param_test: dict, parameters to be tuned
    scoring_method: str, method to be used by the cross-validation to valuate the model
    train: pandas.DataFrame, training data
    predictors: list, labels to be used in the model training process. They should be in the column names of dtrain
    target: str, target variable
    """
    gsearch = GridSearchCV(estimator=alg, param_grid = param_test, scoring=scoring_method,n_jobs=2,iid=False,cv=5)
    gsearch.fit(train[predictors],train[target])
    return gsearch

# get results of the search grid
gs_cls = optimize_num_trees(model_cls,param_test,'roc_auc',train,predictors,target)

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-42-c7419a90cdb1> in <module>()
      1 
      2 # get results of the search grid
----> 3 gs_cls = optimize_num_trees(model_cls,param_test,'roc_auc',train,predictors,target)
      4 

<ipython-input-40-2b76f2ffb87f> in optimize_num_trees(alg, param_test, scoring_method, train, predictors, target)
     57     target: str, target variable
     58     """
---> 59     gsearch = GridSearchCV(estimator=alg, param_grid = param_test, scoring=scoring_method,n_jobs=2,iid=False,cv=5)
     60     gsearch.fit(train[predictors],train[target])
     61     return gsearch

/Users/anaconda/lib/python3.5/site-packages/sklearn/grid_search.py in __init__(self, estimator, param_grid, scoring, fit_params, n_jobs, iid, refit, cv, verbose, pre_dispatch, error_score)
    810             refit, cv, verbose, pre_dispatch, error_score)
    811         self.param_grid = param_grid
--> 812         _check_param_grid(param_grid)
    813 
    814     def fit(self, X, y=None):

/Users/anaconda/lib/python3.5/site-packages/sklearn/grid_search.py in _check_param_grid(param_grid)
    346             if True not in check:
    347                 raise ValueError("Parameter values for parameter ({0}) need "
--> 348                                  "to be a sequence.".format(name))
    349 
    350             if len(v) == 0:

ValueError: Parameter values for parameter (n_estimators) need to be a sequence.

Answer 1

I got a similar error as you:

ValueError: Parameter values for parameter (warm_start) need to be a sequence(but not a string) or np.ndarray. site:stackoverflow.com

The Value for each Key apparently needs to be in array brackets [ ]

My erroneous code:

params = {
    'max_depth': [11],
    'warm_start': True
}

My correct code:

params = {
    'max_depth': [11],
    'warm_start': [True]
}

Answer 2

I met a similar error like you, in the following code:

# optimize n_estimator through grid search
# define range over which number of trees is to be optimized
param_test = {'n_estimators':range(30,151,20)} 

you can change

range(30,151,20) to np.arange(30,151,20)